Selecting between paths based on their existence on the filesystem using default validation at runtime

demo.py

from pathlib import Path
from typing import Annotated

from pydantic import BaseModel, BeforeValidator, Field, FilePath

PathA = Annotated[FilePath, BeforeValidator(lambda _: Path("a.txt"))]
PathB = Annotated[FilePath, BeforeValidator(lambda _: Path("b.txt"))]


class MyModel(BaseModel):
    my_path: PathA | PathB = Field(default="sentinel", validate_default=True)


print("  >> Setup: create only a.txt")
Path("a.txt").touch()
Path("b.txt").unlink(missing_ok=True)

m1 = MyModel()
print(f"Selected: {m1.my_path}")  # Should print a.txt

print()

print("  >> Setup: create only b.txt")
Path("a.txt").unlink(missing_ok=True)
Path("b.txt").touch()

m2 = MyModel()
print(f"Selected: {m2.my_path}")  # Should print b.txt

v1.py

from pathlib import Path
from typing import Annotated

from pydantic import BaseModel, BeforeValidator, Field, FilePath

# Single-expression callable
Candidate = lambda p: Annotated[FilePath, BeforeValidator(lambda _: Path(p))]


class MyModel(BaseModel):
    my_path: Candidate("a.txt") | Candidate("b.txt") = Field(
        default=None, validate_default=True
    )


print("  >> Setup: create only a.txt")
Path("a.txt").touch()
Path("b.txt").unlink(missing_ok=True)

m1 = MyModel()
print(f"Selected: {m1.my_path}")  # a.txt

print()

print("  >> Setup: create only b.txt")
Path("a.txt").unlink(missing_ok=True)
Path("b.txt").touch()

m2 = MyModel()
print(f"Selected: {m2.my_path}")  # b.txt

v2.py

from pathlib import Path
from typing import Annotated

from pydantic import BaseModel, BeforeValidator, Field, FilePath


class Candidate:
    """FilePath that ignores input and resolves to a fixed path. 
    Union of Candidates acts as a fallback chain (first existing wins)."""
    def __class_getitem__(cls, p: str | Path):
        return Annotated[FilePath, BeforeValidator(lambda _: Path(p))]


class MyModel(BaseModel):
    my_path: Candidate["a.txt"] | Candidate["b.txt"] = Field(
        default=None, validate_default=True
    )


print("  >> Setup: create only a.txt")
Path("a.txt").touch()
Path("b.txt").unlink(missing_ok=True)

m1 = MyModel()
print(f"Selected: {m1.my_path}")  # a.txt

print()

print("  >> Setup: create only b.txt")
Path("a.txt").unlink(missing_ok=True)
Path("b.txt").touch()

m2 = MyModel()
print(f"Selected: {m2.my_path}")  # b.txt

v3.py

from pathlib import Path
from typing import Annotated, Union

from pydantic import BaseModel, BeforeValidator, Field, FilePath


def FirstOf(*paths: str | Path):
    # Generator expression: must capture p with p=p
    return Union[tuple(
        Annotated[FilePath, BeforeValidator(lambda _, p=p: Path(p))]
        for p in paths
    )]


class MyModel(BaseModel):
    my_path: FirstOf("a.txt", "b.txt") = Field(default=None, validate_default=True)


print("  >> Setup: create only a.txt")
Path("a.txt").touch()
Path("b.txt").unlink(missing_ok=True)

m1 = MyModel()
print(f"Selected: {m1.my_path}")  # a.txt

print()

print("  >> Setup: create only b.txt")
Path("a.txt").unlink(missing_ok=True)
Path("b.txt").touch()

m2 = MyModel()
print(f"Selected: {m2.my_path}")  # b.txt

v4.py

from pathlib import Path
from typing import Annotated

from pydantic import BaseModel, BeforeValidator, Field, FilePath


def Candidate(p: str | Path):
    # Safe without p=p; closure is unique per call
    return Annotated[FilePath, BeforeValidator(lambda _: Path(p))]


class MyModel(BaseModel):
    my_path: Candidate("a.txt") | Candidate("b.txt") = Field(
        default=None, validate_default=True
    )


print("  >> Setup: create only a.txt")
Path("a.txt").touch()
Path("b.txt").unlink(missing_ok=True)

m1 = MyModel()
print(f"Selected: {m1.my_path}")  # a.txt

print()

print("  >> Setup: create only b.txt")
Path("a.txt").unlink(missing_ok=True)
Path("b.txt").touch()

m2 = MyModel()
print(f"Selected: {m2.my_path}")  # b.txt

Alt. forms, including a rare __class_getitem__, which'd let you do a Literal-like square bracketed hint here